1. Field of the Invention
This invention relates to a method of image formation using a photosensitive screen having a number of fine openings (hereinafter simply referred to as the screen), and more particularly to a method of forming images by modulating a flow of ions several times by the use of one and the same primary electrostatic latent image.
2. Description of the Prior Art
As a typical technique of image formation using the conventional electrophotography, there may be mentioned the direct method such as the electrofax method whereby a developed photosensitive member is directly used as a finished copy, or the indirect method such as the xerography whereby a photosensitive member is used as an intermediate recording medium and the developed image on such photosensitive member is transferred to a transfer medium which is used as the finished copy. Of these two methods, the former, namely, the direct method of image formation employs, as the photosensitive member, a recording member subjected to a special treatment such as coating with a photoconductive substance such as zinc oxide or the like, which causes a decreased brightness and a problem regarding the contrast of the image. Also, said treatment has led to a disadvantage that the recording member gives a somewhat different sense of touch and of weight from those of common plain paper. On the other hand, in the latter method, namely, the indirect method of image formation, image is formed by using plain paper as the transfer medium and this leads to a merit that the resultant copy image is of high contrast and good quality. Nevertheless, with this indirect method, the recording member is brought into contact with the surface of the photosensitive medium during the transfer of toner image to the recording member and further, after the image transfer, the surface of the photosensitive medium is again strongly contacted by cleaning means such as a brush or elastic blade for the purpose of removing any residual toner on the photosensitive medium, so that the surface of the photosensitive medium may gradually become damaged each time it is contacted. This limits the service life of the expensive photosensitive medium, which may also result in higher cost of image formation.
These disadvantages peculiar to the above-described conventional electrophotography are eliminated by the electrophotographic method as disclosed in U.S. Pat. No. 3,713,734. Such electrophotographic method uses a photosensitive screen in the form of a netting or lattice having numberless fine mesh-like openings. Generally described, this method uses the above-mentioned screen, modulates a flow of ions into a form of image through the screen to thereby form an electrostatic latent image on the recording member, and thereafter develops this electrostatic latent image formed on the recording member. That is, this electrophotographic method need not develop and clean the screen which corresponds to the conventionally used photosensistive medium. Thus, the screen itself is never damaged during the image formation and such method is advantageous in that the screen can enjoy a long service life. Particularly, the method disclosed in our U.S. Application Ser. No. 480,280 has been successful in improving the durability of the screen and utilizing a once formed primary electrostatic latent image more repetitively than before to form images (hereinafter referred to as retention copying). Herein, this method covered by our above-mentioned application need not be described in detail but will only generally be described below. First, the screen is constructed by covering an electrically conductive member, which is a substrate, with a photoconductive member and then with an insulating member, in such a manner that the conductive member is exposed at one side surface thereof. Image formation is effected thus: a primary electrostatic latent image is formed on the screen and this primary latent image is used to modulate an ion flow applied to a chargeable member, thereby providing a secondary electrostatic latent image on the chargeable member. As the chargeable member, use may be made of either electrostatic recording paper or a recording medium in the form of a drum (insulating drum) having an insulating layer which is less expensive than that of the conventional photosensitive medium. Where the electrostatic recording paper is used, it is directly developed and fixed by well-known means for utilization. In contrast, where the insulating drum is used, the secondary electrostatic latent image formed on the drum is one developed, and then transferred to another recording member such as plain paper or the like, whereafter the latter is fixed for utilization. Thus, the insulating drum can be rendered available for repetitive use by removing residual toner therefrom after the image transfer and moreover, the resin material forming the insulating layer has excellent wear-proof durability chacteristics.
When the insulating drum is used as described, a voltage must be applied between the screen and the recording member to attract the modulated ion flow toward the insulating drum so that the ion flow may be directed to the insulating drum side. However, the residual toner, remaining on the insulating drum after the image transfer, is attracted toward the screen due to the electric field induced by said voltage application which is acting adjacent to the screen and between the screen and the insulating drum. Of course, most of such residual toner on the insulating drum is removed by cleaning means after the image transfer, but a slight quantity of the residual toner which failed to be removed by the cleaning means goes to stick to the screen which is provided with no cleaning means. With lapse of time, the quantity of the toner sticking to the screen increases and as the result, the screen suffers from a reduced insulating property of the portion thereof which should provide electrical insulation, and/or the openings of the screen become clogged, thus rendering good modulation of ions impossible. This phenomenon will be more fully considered in connection with an example of the conventional apparatus which will hereinafter be described.